新型钙钛矿铁电光伏材料的理论设计

Solar energy is a clean and renewable energy resource. Solar energy storage can be achieved by photovoltaic effect, where the photovoltaic materials absorb the sun light and convert solar energy into electric energy. Among many photovoltaic materials, ferroelectric photovoltaic material, where a single-phase ferroelectric material is used for light absorbing, displays anomalous photovoltaic effect and has been extensively investigated. However, most solid oxide ferroelectric materials are wide-gap insulators that only absorb very little of the solar spectrum, leading to the low photocurrent output. Therefore, designing the new semiconducting ferroelectrics with small band gaps, which are more effective for sun light absorption, are required. In this proposal, we propose to use first principles calculations to study the electronic structures and ferroelectric properties of the Pervoskite ferroelectrics, and design new semiconducting Perovskite ferroelectrics for photovoltaic applications. Taking the advantage of the latest research progress made on ferroelectric materials, we will apply the hybrid improper ferroelectricity for ferroelectric photovoltaic material design. We will choose some special non-ferroelectric Perovskite photovoltaics, and modify them into the novel ferroelectrics with hybrid improper ferroelectricity using the structural engineering. In other words, we will induce polarization in non-ferroelectric photovoltaic materials, and obtain new ferroelectric photovoltaics. We propose a new ferroelectric photovoltaic material design strategy, which is totally different from the conventional cation doping approach. Our designed ferroelectric photovoltaic materials will have the stable polarization at room temperature, semiconducting electronic properties, and can be created in experiment by standard synthesis technique. Our proposal of discovering new Pervoskite ferroelectric photovoltaics should stimulate significant materials research in this area and also provide the theoretical guideline for experimental design, synthesis and characterization of Pervoskite ferroelectric photovoltaic materials.

为了有效地利用清洁、可再生的太阳能，需要通过光伏材料吸收太阳光，将太阳能转化成电能存储起来。在诸多光伏材料中，具有铁电自发极化的铁电光伏材料因奇特的性质而备受关注。但是绝大多数的铁电材料是宽带隙的绝缘体，无法有效地吸收太阳光，因此急需设计出更多的具有窄带隙、半导体特性的铁电光伏材料。在本项目中，我们将利用第一性原理计算方法，通过对钙钛矿铁电材料电子结构和铁电性质的研究，来设计新型钙钛矿铁电光伏材料。我们将借鉴铁电领域的最新研究进展，选择特定的非铁电钙钛矿光伏材料，将其设计、改造成具有杂化反常铁电性的铁电光伏材料，实现光伏材料铁电化。这不同于传统的通过离子掺杂使铁电材料半导体化的设计思想。通过本研究项目，我们将设计出具有稳定铁电极化、半导体特性且易于实验合成的新型铁电光伏材料。本项目不仅会为铁电光伏材料的设计提供新的思路，还会为将来实验上合成、表征新型钙钛矿型铁电光伏材料提供理论依据。

具有稳定的铁电极化，能够高效吸收太阳光的窄带隙、半导体特性的铁电光伏材料可利用铁电光伏效应实现较高的光电能量转换效率。有别于传统的通过离子掺杂使得绝缘性钙钛矿铁电氧化物半导体化的铁电光伏材料设计方案，在本项目中，我们创新性地提出了“钙钛矿半导体材料铁电化”的全新设计理念。为此，我们选择了若干种具有正交Pnma结构的顺电钙钛矿硫化物、氮氧化物和碘化物。根据铁电极化与a-a-c+八面体旋转模式之间的耦合关联，我们通过结构调制，将上述顺电钙钛矿半导体材料设计、改造成具有杂化反常铁电性的新型钙钛矿铁电光伏材料：即Ruddlesden–Popper钙钛矿硫化物A3B2S7 (A = Ca，Sr，Ba; B = Zr，Hf)，氮氧化物Ca3Nb2O5N2和A位有序的双钙钛矿(Cs/Rb)Sn2I6。通过本项目的研究，我们首次在钙钛矿硫化物、氮氧化物和碘化物体系中实现了稳定的铁电极化。同时我们的计算模拟结果还表明，我们所设计的钙钛矿铁电光伏材料均为直接带半导体(带隙在1.5 ~2.2 eV之间)，可以有效的吸收太阳光，有望实现较高的能量转换效率。目前实验上已成功合成了Ruddlesden-Popper钙钛矿硫化物Ba3Zr2S7，证实了我们的理论预测。我们还通过开展第一性原理计算和对称性分析，确立了上述几种钙钛矿铁电光伏材料的带隙、铁电极化与钙钛矿结构参数(如钙钛矿容限因子等)的依赖关系，为实现对钙钛矿铁电光伏材料性质的可控调制和设计高性能铁电光伏材料提供了依据。此外，我们还探索了实验上已合成但尚未未被关注的半导体性的铁电光伏材料，发掘出一系列具有稳定的铁电极化和窄带隙的铁电光伏材料，如双钙钛矿CaMnTi2O6，Sn2P2S6-xSe6x铁电固溶体和NbOI2二维层状铁电材料。通过开展实验合作，我们还成功设计、制备出一种新型钙钛矿铁电光伏材料Sr1-xBi2+xNb2-xNixO9-x，并对其光伏性能进行了表征。通过本项目的研究，我们不仅提出了设计铁电光伏材料的新理念，同时还为今后研究和表征Ruddlesden-Popper卤族钙钛矿体系中所出现的奇特铁电性质提供了理论依据。